Yes, your dog is wagging its tail. But there’s more to it than that. Dogs are like onions: they have layers. The tail wag is a nuanced expression. According to researchers led by Giorgio Vallortigara, a neuroscientist at the University of Trento in Italy, the direction of the wag indicates a dog’s emotions.

Dogs wagging to the right are relaxed. Tails heading left indicate stress. That’s because the direction of tail movement indicates whether the dog is using primarily the left side of his brain or the right side of his brain.

Prof. Vallortigara and his team conducted their study by examining the movements of 43 dogs that were hooked up to heart monitors:

To find out if other dogs responded to the direction of tail wags, the researchers recruited 43 dogs of various breeds and showed them videos of another dog or a digitized silhouette of a dog with its tail wagging left or right.

The observing dogs were fitted with a vest to measure their heart rate, and their behaviors were filmed and analyzed.

When dogs looked at tails wagging to the left, their heart rate increased and they showed more signs of stress and anxiety. The dogs were more relaxed when they saw tails wagging to the right.